Some basic facts about black boxes (a misnomer, by the way; they're generally painted bright red to help rescuers find them after a crash):

• The Cockpit Voice Recorder typically records two hours of audio from the flight crew in a continuous loop. The last two hours of conversation, either between pilots or over the radio, should be preserved.

• The Flight Data Recorder often records 400 different aspects of airplane performance -- from airspeed and cabin pressure to engine speed, movements of the rudder, flaps and other control surfaces.

• The two boxes are tough. They are designed to withstand 3,400 times the force of gravity on impact. The boxes on Air France 447, an Airbus 330 that entered service in 2005, had all-digital components, without moving parts to record data.

• The recorders on the Airbus 330 weigh about 13 pounds, and are firmly bolted in place near the tail of the aircraft. They are not designed to float. There have been suggestions that they be built to break free and float to the surface if a plane crashes in water, but that would require a major redesign.

• In the ocean, where radio signals do not carry, they have an "underwater locator beacon," which sends out an audible "ping" so they can be located.

• There's something of a clock ticking: the locator signals are certified to keep working for 30 days, though Honeywell, a major manufacturer of the boxes, say they'll probably last longer.

• More worrisome: the signal may be hard to detect if the boxes are in rough underwater terrain. If the sea floor is flat, they should easily be detected from the 9,000-foot depths where the plane is believed to have crashed. If not, they may be harder to find.

"The terrain at the bottom of the ocean is not uniform, and apparently in this area there are submerged mountains and canyons and all sorts of stuff," said Duncan Schofield, principal engineer for flight recorders at Honeywell Aerospace. "If it is stuck in a crevice somewhere we could never hear it, because the sound would be blocked by the features of the bottom."

Air France 447 Crash: The Black Boxes

The black boxes should be able to survive in waters of 20,000 feet, far deeper than the mid-Atlantic region where Air France 447 was believed lost.

But the head of France's accident investigation agency, Paul-Louis Arslanian, said Wednesday that he's "not optimistic" the black boxes from the Air France jet will ever be recovered.

The Airbus A330 with 238 aboard fell from the sky over the Atlantic Sunday night on a flight from Rio de Janeiro to Paris. Officials said the plane ran into an area of strong thunderstorms, and automated messages from the plane suggest that the aircraft broke apart at altitude.